In our previous U.S. Pat. Nos. 3,293,439 and 3,716,716 there are disclosed electrical control arrangements for enabling the automatic centering of a hole location on a printed circuit board precisely at designated coordinate points for drilling. In these systems, a table is arranged to be moved in the direction of X and Y orthagonal axes by X and Y servo motors. This table supports the printed circuit board and also an art work positive film upon which the exact hole locations to be drilled on the board are designated. A light projected up through the film onto a photo cell on the other side of the film provides an arrangement for detecting the hole location. Exact centering is accomplished by providing a four element electronically switched photo-sensitive resistance element, the four elements being spaced about the exact center such that when the signals across the photo sensitive resistance are all equal, exact centering has been accomplished. Another means as discussed in the afore-mentioned patents utilizes a rotating photo cell for time sharing sampling about the periphery of the art work to thereby locate the exact center when the output signal is uniform over a 360.degree. rotation. In both instances, the generated signals are amplified and utilized to drive the servo motors minute distances to effect the exact positioning of the circuit board itself for proper drilling.
Other centering systems utilize a projection means wherein the art work target is magnified several times and projected onto a screen having orthagonal X and Y axes the intersection point of which constitutes the exact center. The table and art work can then be shifted slightly while viewing the screen to effect an exact registration of the projected magnified target with the center or intersection point of the orthagonal axes.